Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760879AbYJIT0r (ORCPT ); Thu, 9 Oct 2008 15:26:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755344AbYJIT0h (ORCPT ); Thu, 9 Oct 2008 15:26:37 -0400 Received: from wr-out-0506.google.com ([64.233.184.239]:19382 "EHLO wr-out-0506.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759456AbYJIT0e (ORCPT ); Thu, 9 Oct 2008 15:26:34 -0400 Message-ID: <48EE5AE1.5030002@codemonkey.ws> Date: Thu, 09 Oct 2008 14:26:25 -0500 From: Anthony Liguori User-Agent: Thunderbird 2.0.0.17 (X11/20080925) MIME-Version: 1.0 Newsgroups: gmane.linux.network,gmane.linux.kernel,gmane.linux.kernel.virtualization To: Mark McLoughlin CC: Herbert Xu , Rusty Russell , linux-kernel@vger.kernel.org, virtualization@lists.osdl.org, netdev@vger.kernel.org Subject: Re: [PATCH 2/2] virtio_net: Improve the recv buffer allocation scheme References: <1223494499-18732-1-git-send-email-markmc@redhat.com> <1223494499-18732-2-git-send-email-markmc@redhat.com> <200810091155.59731.rusty@rustcorp.com.au> <20081009153035.GA21542@gondor.apana.org.au> <1223574013.13792.23.camel@blaa> In-Reply-To: <1223574013.13792.23.camel@blaa> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1708 Lines: 41 Mark McLoughlin wrote: > > Also, including virtio_net_hdr in the data buffer would need another > feature flag. Rightly or wrongly, KVM's implementation requires > virtio_net_hdr to be the first buffer: > > if (elem.in_num < 1 || elem.in_sg[0].iov_len != sizeof(*hdr)) { > fprintf(stderr, "virtio-net header not in first element\n"); > exit(1); > } > > i.e. it's part of the ABI ... at least as KVM sees it :-) This is actually something that's broken in a nasty way. Having the header in the first element is not supposed to be part of the ABI but it sort of has to be ATM. If an older version of QEMU were to use a newer kernel, and the newer kernel had a larger header size, then if we just made the header be the first X bytes, QEMU has no way of knowing how many bytes that should be. Instead, the guest actually has to allocate the virtio-net header in such a way that it only presents the size depending on the features that the host supports. We don't use a simple versioning scheme, so you'd have to check for a combination of features advertised by the host but that's not good enough because the host may disable certain features. Perhaps the header size is whatever the longest element that has been commonly negotiated? So that's why this aggressive check is here. Not to necessarily cement this into the ABI but as a way to make someone figure out how to sanitize this all. Regards, Anthony Liguori -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/